How Holography Works: Explained

[kelvin490]

To make a hologram a film is exposed to an incident plane wave and wave from the object to record the interference pattern on the film. The principle is commonly explained in a way like that in p.1212 of "University Physics" ( https://books.google.com.hk/books?id=7S1yAgAAQBAJ&pg=PA1211&lpg=PA1211&dq )

What I don't understand is why a 3D image can be made by shining a plane wave through the film. The film is grating so at some points constructive interference can produce the point representing the object. But why the overall wave is diverged (show in 36.29b, p.1212) ?

 

[Andy Resnick]

It's not a 3-D image; it just appears that way because the reconstructed wavefront is (essentially) identical to that produced when light scatters of a 3D object.

Holography is a coherent imaging process- both amplitude and phase of the wavefront are recorded at each point on the holographic film; the complex-valued field is converted into intensity via interference with a reference beam. This is also why a reference beam is required to generate the holographic image; use an illumination beam that differs from the reference (say, not a plane wave) and your holographic reconstruction will also appear different than the original object scene.

 

[sophiecentaur]

both amplitude and phase of the wavefront are recorded at each point on the holographic film;

I have a bit of a problem with this idea. The film can only record the intensity (one value) of a three dimensional diffraction pattern at one point on its plane. It seems that this is enough to reconstruct the original object (with a certain resolution) but you are surely losing something in the process. (?)

 

[blue_leaf77]

To be precise, it's the phase difference between the object and reference waves which is recorded (or in another term, encoded).

 

[sophiecentaur]

To be precise, it's the phase difference between the object and reference waves which is recorded (or in another term, encoded).

The effect of amplitude must be in there somewhere. Perhaps it gets a bit lost, which could be why holograms often look a bit low constast.

 

[ZapperZ]

The effect of amplitude must be in there somewhere. Perhaps it gets a bit lost, which could be why holograms often look a bit low constast.

But this is often more to do with the brightness of the source that is "replaying" the image. It is certainly true for transmission holograms.

The most important aspect of holograms is that it "replays" both the image and the phase of the image. The latter is perceived by our eyes as depth.

Zz.

 

[Andy Resnick]

I have a bit of a problem with this idea. The film can only record the intensity (one value) of a three dimensional diffraction pattern at one point on its plane. It seems that this is enough to reconstruct the original object (with a certain resolution) but you are surely losing something in the process. (?)

Recording the intensity of an interferogram is the conceptual breakthrough for holography. Because the actual physical recording is the real-valued intensity, illuminating a hologram generates 2 images, complex conjugates of each other, corresponding to the real and virtual images. Similarly, one can illuminate a hologram with either the reference beam or the complex conjugate of the reference beam. Goodman's "Fourier Optics" text is *the* reference for learning the principles of 'wavefront reconstruction imaging' (holography).